The present invention relates generally to semiconductor devices and, more particularly, to the formation of electrical contacts to conductive elements in semiconductor integrated circuits.
One technique for forming a metallization pattern on an integrated circuit is to etch a conductor pattern into an insulating layer to form an inlaid pattern or grooves. A metal layer then is deposited to fill the etched grooves. Typically, the metal layer not only fills the grooves, but also covers the entire semiconductor wafer. The excess metal over the wafer surface is removed either by a chemical-mechanical polishing process or by an etchback process. In-laid conductors, thus, are formed in the insulating layer. Such a process also is known as a damascene process.
Vias are needed to connect different metallization layers. In damascene processes, the vias are formed in the insulating layer and subsequently are filled with metal. The excess metal over the wafer surface is removed. Formation of the vias is followed either by a standard metallization process or by a damascene conductor layer as described above. Forming the vias and conductors separately is referred to as a single damascene process. According to a simpler process, the vias and the metallization patterns are formed in the insulating layer, followed by a single metal filling and excess metal removal process. Formation of the vias and conductors together is referred to as a dual damascene process.
The dual damascene process is used, for example, to form multi-level signal lines of metal for a multi-layer substrate on which the semiconductor devices are mounted. Thus, a first or lower metal interconnect line can be electrically in contact with a doped region of the substrate of an integrated circuit device. One or more metal interconnections are formed between the first metallization level and other portions of the device or to structures external to the integrated circuit device. Those interconnections are accomplished, in part, by the second level of wiring lines.
In a standard dual damascene process, the insulating layer is coated with a resist and exposed to a first mask with an image pattern of via openings. The pattern is anisotropically etched in the upper half of the insulating layer. After removal of the patterned resist, the insulating layer is coated with a resist and exposed to a second mask with an image pattern of conductive lines in alignment with the vias. During the anisotropic etch of the openings for the conductive lines, the via openings are simultaneously etched in the lower half of the insulating layer. After etching is completed, the resist is stripped for example, using an oxygen plasma. The vias and the grooves for the conductors then are filled with metal.
Aluminum (Al) is often used for the metallization. However, metals such as copper (Cu) are desirable for use as conducting lines because they have good electrical conductivity and are resistant to electro-migration which can occur in Al. Therefore, Cu is an attractive replacement for Al due to its low cost and ease of manufacturing. Nevertheless, one difficulty in using Cu for conducting lines results from the high susceptibility of Cu to oxidation. As noted above, the photoresist used for patterning typically is removed at the end of processing by heating it in a highly oxidizing environment, such as an oxygen plasma, to convert the photoresist into an easily removable ash. During such ashing processes, the lower Cu conductive line, which is subjected to the oxidizing ambient, will become oxidized, thereby, causing a deterioration in the electrical properties of the metal contacts. Such problems are not limited to the fabrication of dual damascene structures or the use of Cu as the conductive material. However, such a structure highlights the type of problems that may be encountered when a contact needs to be made to a conductive element that can become oxidized when exposed to oxygen.
One technique for avoiding the oxidation of the lower Cu metallization layer is to employ an organic solvent to remove the photoresist. However, such solvents are expensive and are hazardous to the environment. Accordingly, alternative techniques that avoid the foregoing problems are desirable.